Creating and Optimizing Interfaces for Electric-Field and Photon-Induced Charge Transfer

We create and optimize a structurally well-defined electron donor–acceptor planar heterojunction interface in which electric-field and/or photon-induced charge transfer occurs. Electric-field-induced charge transfer in the dark and exciton dissociation at a pentacene/PCBM interface were probed by <i>in situ</i> thickness-dependent threshold voltage shift measurements in field-effect transistor devices during the formation of the interface. Electric-field-induced charge transfer at the interface in the dark is correlated with development of the pentacene accumulation layer close to PCBM, that is, including interface area, and dielectric relaxation time in PCBM. Further, we demonstrate an <i>in situ</i> test structure that allows probing of both exciton diffusion length and charge transport properties, crucial for optimizing optoelectronic devices. Competition between the optical absorption length and the exciton diffusion length in pentacene governs exciton dissociation at the interface. Charge transfer mechanisms in the dark and under illumination are detailed

Light-Tunable Polarity and Erasable Physisorption-Induced Memory Effect in Vertically Stacked InSe/SnS₂ Self-Powered Photodetector

van der Waals heterojunctions with tunable polarity are being actively explored for more Moore and more-than-Moore device applications, as they can greatly simplify circuit design. However, inadequate control over the multifunctional operational states is still a challenge in their development. Here, we show that a vertically stacked InSe/SnS2 van der Waals heterojunction exhibits type-II band alignment, and its polarity can be tuned by an external electric field and by the wavelength and intensity of an illuminated light source. Moreover, such SnS2/InSe diodes are self-powered broadband photodetectors with good performance. The self-powered performance can be further enhanced significantly with gas adsorption, and the device can be quickly restored to the state before gas injection using a gate voltage pulse. Our results suggest a way to achieve and design multiple functions in a single device with multifield coupling of light, electrical field, gas, or other external stimulants

Tunable Exciton Dissociation and Luminescence Quantum Yield at a Wide Band Gap Nanocrystal/Quasi-Ordered Regioregular Polythiophene interface

A comprehensive understanding of the effect of polymer chain aggregation-induced molecular ordering and the resulting formation of lower excited energy structures in a conjugated polymer on exciton dissociation and recombination at the interface with a wide-bandgap semiconductor is provided through correlation between structural arrangement of the polymer chains and the consequent electrical and optoelectronic properties. A vertical diode-type photovoltaic test probe is combined with a field effect current modulating device and various spectroscopic techniques to isolate the interfacial properties from the bulk properties. Enhanced energy migration in the quasi-ordered (poly­(3-hexylthiophene)) (P3HT) film, processed through vibration-induced aggregation of polymer chains in solution state, is attributed to the presence of the aggregation-induced interchain species in which excitons are allowed to migrate through low barrier energy sites, enabling efficient iso-energetic charge transfer followed by the downhill energy transfer. We discovered that formation of nonemissive excitons that reduces the photoluminescence quantum yield in the P3HT film deactivates exciton dissociation at the donor (P3HT) close to the acceptor (ZnO) as well as in the P3HT far away from the ZnO. In other words, exciton deactivation in its film state arising from the quasi-ordered structural arrangement of polymer chains in solution is retained at the donor/acceptor interface as well as in the bulk P3HT. Effect of change in the highest occupied molecular orbital level and the resulting energy band bending at the P3HT/ZnO interface on exciton dissociation is also discussed in relation to the presence of vibration-induced aggregates in the P3HT film

Enhanced NO₂ Sensitivity of Vertically Stacked van der Waals Heterostructure Gas Sensor and Its Remarkable Electric and Mechanical Tunability

Nanodevices based on van der Waals heterostructures have been predicted, and shown, to have unprecedented operational principles and functionalities that hold promise for highly sensitive and selective gas sensors with rapid response times and minimal power consumption. In this study, we fabricated gas sensors based on vertical MoS2/WS2 van der Waals heterostructures and investigated their gas sensing capabilities. Compared with individual MoS2 or WS2 gas sensors, the MoS2/WS2 van der Waals heterostructure gas sensors are shown to have enhanced sensitivity, faster response times, rapid recovery, and a notable selectivity, especially toward NO2. In combination with a theoretical model, we show that it is important to take into account created trapped states (flat bands) induced by the adsorption of gas molecules, which capture charges and alter the inherent built-in potential of van der Waals heterostructure gas sensors. Additionally, we note that the performance of these MoS2/WS2 heterostructure gas sensors could be further enhanced using electrical gating and mechanical strain. Our findings highlight the importance of understanding the effects of altered built-in potentials arising from gas molecule adsorption induced flat bands, thus offering a way to enhance the gas sensing performance of van der Waals heterostructure gas sensors